The Operative Dentistry department at the University of North Carolina is the largest clinical research center for posterior composites and maintains the largest database of information on composite research restorations (9 projects involving 26 materials). We have recently presented evidence that (a) the wear rate of posterior composites decreases exponentially with time, (b) its prevalence and severity by site is 1st molars> 2nd molars> 2nd premolars> 1st premolars, and (c) wide or complex restorations are much more susceptible. Wear occurs on restorations at occlusal contact areas but is predominantly at contact free ares. Our results support the following "protection" hypothesis. Contact free wear is produced intraorally by > 0.1 um abrasive particles in food that selectively abrade the resin matrix of composites. Wear is limited by protection from closely spaced filler particles and from cavity preparation walls. Local filler particle spacing controls microscopic wear protection while cavity preparation size and location control macroscopic wear protection. This hypothesis appears to explain published wear results from most studies and sources. The objective of this proposal is to test this hypothesis with clinical data on wear rates and patterns from different tooth surfaces (occlusal, ridges and grooves, facial and lingual, interproximal), different tooth classes (molars, premolar, incisors), different composite types (conventional, hybrid, microfills), and different intraoral times (0- 0.5, 0.5-5, and 5-10 years). Clinical data on wear rates and patterns will be compared to laboratory data from modified DeGee abrasion tests to simulate clinical wear. Filler particle distribution, sizes, and interparticle spacing will be characterized for composites. Long term wear rate data is available from previous studies. Initial wear rate behaviors will be studied by inserting three posterior composites. Wear patterns will be characterized from epoxy replicas with the SEM. Local wear rates will be determined at margins with the Leinfelder scale, M-L scale, and SEM measurements. Information from clinical evaluations, cast ratings, and replica analyses will be statistically analyzed to compare in vitro wear and test the "protection" wear theory.